Phagocytosis of Trypanosoma congolense by macrophages : the role of IgM antibody to variant surface glycoprotein (VSG)

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Trypanosoma congolense is a single-cell blood parasite and an important pathogen causing African trypanosomiasis, also called n’gana, in livestock. N’gana in cattle is a chronic disease associated with anemia, cachexia and increased susceptibility to secondary infections. Infection of mice can be used as an experimental model to study the host-parasite relationship. As determined by their survival time, BALB/c mice are highly susceptible to T. congolense infection, whereas C57BL/6 mice are relatively resistant. The surfaces of African trypanosomes are covered with a layer of a single species of glycoprotein, called variant surface glycoprotein (VSG). Production of antibodies to the VSG of African trypanosomes is one of the major immune responses leading to control of parasitemia. The reaction of antibodies with VSG of trypanosomes, for presently unknown reasons, predominantly activates the alternative complement pathway rather than the classical pathway of complement. IgM antibodies are the first and predominant class of anti-trypanosomal antibodies in infected animals. Antibody-mediated phagocytosis of T. congolense by macrophages is considered a major mechanism of control of parasitemia, besides antibody/complement-mediated lysis and cytotoxic effect by macrophage-derived nitric oxide (NO). The receptor(s) on macrophages that recognizes IgM antibody-coated trypanosomes and enables their phagocytosis is unknown. Interaction of antibodies with the VSG of trypanosomes not only causes phagocytosis of trypanosomes by macrophages, but also leads to the release of sVSG from the trypanosomes. sVSG has been found to modulate various functions of the host: induction of polyclonal B cell activation and modulation of macrophage functions, such as the induction of TNF-á synthesis and the inhibition of IFN-ã-induced nitric oxide production. The objectives of this thesis are:
1) to test whether CR3 (Mac-1; CD11b/18) is involved in IgM anti-VSG-mediated phagocytosis of T. congolense by macrophages
2) to test the effects of anti-VSG antibody and complement on the release of soluble VSG from T. congolense 1) When the trypanosomes were incubated with IgM anti-VSG antibody and fresh mouse serum, fragments of complement component C3 were found to be deposited onto Trypanosoma congolense . Thus, it was assessed whether complement receptor CR3 (CD11b/CD18; receptor for iC3b) might be involved in IgM anti-VSG mediated phagocytosis of T. congolense . In the presence of fresh mouse serum, there was significantly and markedly less phagocytosis of IgM-opsonized T. congolense by CD11b-deficient macrophages compared to phagocytosis by normal macrophages (78% fewer T. congolense were ingested per macrophage). There also was significantly less TNF-á (38% less), but significantly more NO (63% more) secreted by CD11b-deficient macrophages that had engulfed trypanosomes than by equally treated normal macrophages. It was concluded that CR3 is the major, but not the only, receptor involved in IgM anti-VSG-mediated phagocytosis of T. congolense by macrophages. It was further concluded that signaling via CR3, associated with IgM anti-VSG-mediated phagocytosis of T. congolense , either directly or indirectly, enhances synthesis of disease-producing TNF-á and inhibits the synthesis of parasite-controlling NO. 2) This investigation revealed that there was more sVSG released from T. congolense by interaction with IgM anti-VSG than by interaction with equal amounts of IgG2a anti-VSG. The release of sVSG occurred in an antibody dose-dependent pattern. It was also found that IgM anti-VSG, after interacting with the surface of T. congolense , formed soluble immune complexes with released sVSG. The results also showed that antibody-induced release of sVSG can occur without complement, but is enhanced by complement. It was further tested whether fresh sera from either relatively resistant C57BL/6 mice or highly susceptible BALB/c mice, which differ in their complement cascade, had different effects on the release of sVSG from T. congolense . The results showed that antibody-induced shedding of sVSG was higher in the presence of fresh C57BL/6 serum than in the presence of fresh BALB/c serum. All these data suggest that the concentration of anti-VSG antibody, antibody class and source of complement can affect the release of sVSG from T. congolense .